High audio quality either in a studio environment or in a live performance environment is difficult to achieve especially where audio processing devices are chained together or indeed where all cabling is run to a "patch bay" or to individual pieces of equipment. In order to facilitate the recording or performance of music or vocal works, or indeed anything in which audio quality is important, in the past it has been the practice to run multiple audio cables from various audio sources directly to a patch bay. The signals from the patch bay can be routed to audio processing equipment which may include simple mixing circuits, distribution networks or equipment which introduces special effects. Maintaining fidelity in such systems is dependant in large part, on the type of cabling utilized and the proper installation of all equipment and cables according to well-established audio engineering practice.
In general, so-called analog snakes provide the required signal transmission. The audio snake is a heavy cable containing multiple shielded conductors and some unshielded lines, with the audio snake weighing as much as 150 pounds for a large installation. Moreover, due to the multi-conductor internal audio cabling, sophisticated large multi-pin connectors are used at either end of the snake.
In general, the problem with providing a link between multiple audio sources and a remotely located processor starts with the problem of field use of audio equipment and analog transmission of signals therebetween. For performances such as concerts, field remote taping and studio work, these situations involve as many as forty or fifty audio sources in the recording or performance of the audio work. Typically in these situations there are large numbers of sources of audio signals and other types of signals that are required to be transmitted from a central point to distributive points, from distributive points to a central point, or from one set of distributive points to another set of distributive points.
With all of the audio signals and control signals to be transmitted, large numbers of cables, each carrying an analog signal, have been utilized in a brute force approach, to transmit signals between different sites and a centralized processing unit within a control booth, for instance. The problem with such a system is that it is expensive, and suffers from the large number of cables which are routed in a piecemeal fashion across the floor.
More importantly, these signals typically have to travel long distances which results in a substantial amount of signal degradation. It is also very inconvenient to lay down large numbers of cables quickly and this is oftentimes a requirement in field usage. Additionally, with the advent of the above-mentioned audio snake, the cabling is not only expensive and heavy, but when numbers of side-by-side cables are encased within a single sheath, cross talk is prevalent, along with the pick up of AC hum. Further, other signals than audio signals are frequently routed through the audio snake such as control signals for the control of lights, lasers or other special effects devices. Moreover, return audio is routinely routed through the snake back to monitors at the stage or to speakers on the stage. It is therefore difficult to transmit undegraded signals having a classical bandwidth of 20 Hz to 20 kHz over long distances and with acceptable purity.
While the audio snake has in part taken the place of the routing of multiple wires from multiple audio sources directly to the patch bay, audio snakes may be 1-2" in diameter, and may carry as many as 50 audio cables side by side. As noted, each snake is expensive, as are the relatively complex connectors which are required at either end of the audio snake to attach the cable to connector boxes.
Frequently what is found in some installations is that low level signals and the higher level signals are mixed in the same snake cable which provides an opportunity for cross talk between the two sets of signals. If one has signals from microphones mixed in with signals for instance from power amplifiers, one can obtain regenerative feedback oscillation because one can have outputs being cross talked into the inputs and this can cause a continuous recirculation of the signal which leads to oscillation and other instabilities. Crosstalk as used herein refers to the mixing of a signal in one channel into a signal transmitted in another channel, with this problem being especially acute in studios where high quality is absolutely mandatory and channel separation is one of the requisites.
It will be appreciated that the signal strength through each of the channels of the audio snake can vary anywhere from a very low level microphone output which is not recommended but is often done, to high levels associated with speaker outputs. The mixture of different levels can be troublesome even for line output levels on the order of 1 volt RMS which is usually available after a microphone output has been amplified.
With respect to line loss, it has been recommended that for 250 ohm microphones the frequency losses above 10 kHz are significant if the cable length is on the order of 200 feet. The 10 kHz cutoff is significantly in the audible range, and cable lengths of over 200 feet are not uncommon, especially for performances on stages and in auditoriums or athletic fields. This causes significant high end signal degradation.
Degradation in general takes three forms. It can be the addition of noise, the introduction of non-linearities, or it can be the droppage of the higher frequency components of an amplified signal. Additional noise, including cross talk and ground loops, can occur at all frequencies but is usually in the form of either 60 Hz hum or high frequency interference, such as RF interference or spurious oscillations.
In order to minimize cross talk, traditional analog cables have utilized shielding in order to isolate the signals carried by the cables within the bundle. In less expensive audio snakes there is only one main shield. This causes considerable cross talk problems because there is considerable unwanted interference between signals transmitted along the unshielded cables which are bundled together.
The ground loop problem is also a large problem in the field and even in studio applications where very low level signals have hum introduced because of improper grounding problems. This can be even more magnified when there is no DC isolation between the signals.
In order to minimize audio degradation in a large concert set up, instead of plugging a microphone directly into the audio snake, one can pre-amplify the microphone signal to boost the signal so that it can be sent further distances. However, the amplification in and of itself causes problems. These problems include the aforementioned noise, and the problems of ground loops, with the ground loop problem being severe when amplifiers are improperly grounded, which is frequently the case in non-permanent, and some permanent set-ups. The ground loop problems can be solved by the utilization of isolation transformers at either end of cable. However, the utilization of isolation transformers in itself introduces problems of non-linearity and saturation.
Thus, in the past in order to solve the problems associated with multiple audio signals in either stage or studio applications, relatively creative analog approaches have been utilized in order to provide signal transmission from the audio source to a remote audio processing unit. However creative, all analog approaches suffer from the above-mentioned problems.